1. Field of the Invention
This invention relates to an image-forming apparatus and an image-forming method, and more particularly to an image-forming apparatus and an image-forming method which are used to form a high-quality image by electrophotography.
2. Related Background Art
As techniques for element members used in electrophotographic photosensitive members, various materials are proposed, as exemplified by selenium, cadmium sulfide, zinc oxide, phthalocyanine and amorphous silicon (hereinafter "a-Si"). In particular, non-single-crystal deposited films containing silicon atoms as a main component as typified by a-Si films, e.g., amorphous deposited films such as an a-Si film compensated with hydrogen and/or halogen (e.g., fluorine or chlorine), are proposed as photosensitive members having a high performance and a high durability and free of environmental pollution. Some of these have been put into practical use. Japanese Patent Application Laid-Open No. 54-86341 discloses a technique of an electrophotographic photosensitive member whose photoconductive layer has been formed chiefly of a-Si.
As a process for forming electrophotographic amorphous silicon deposited films, a plasma CVD (chemical vapor deposition) process, which is a process of forming a thin-film deposited film on a substrate of glass, quartz, heat-resistant synthetic resin film, stainless steel or aluminum by decomposing material gases by glow discharge using direct current or high frequency (RF or VHF) power or microwaves, has been put into practical use in a very advanced state. Apparatus therefor are also proposed in variety.
In addition, in recent years, there is an increasing demand for the improvement in film quality and processing capability, and various means therefor are on study.
In recent years, there is a report on plasma CVD making use of a high-frequency power source, using a parallel-plate plasma CVD system (Plasma Chemistry and Plasma Processing, Vol.7, No.3, 1987, pp.267-273). This report shows a possibility that deposition rate can be improved by making discharge frequency higher than conventional 13.56 MHz, without damaging the performance of deposited films, and attracts notice. This report on making discharge frequency higher is also made in respect of sputtering, and studies thereon have been widely made in recent years.
In electrophotographic process employing such photosensitive members, the step of transferring to a recording medium such as paper a toner image formed on the surface of a photosensitive member is repeated to form images, where residual toner remaining on the photosensitive member surface must be completely removed every time the toner image is transferred. As a cleaning assembly therefor, an assembly is available which is so constructed that a rubber blade made of an elastic material such as urethane rubber is brought into pressure contact with the photosensitive member surface to remove the residual toner. This cleaning assembly has a simple construction, is small-sized and of low-cost and has a good function to remove toner, and hence has been put into practical use in low-speed machines and to high-speed machines.
Various studies are made on a technique for reducing friction between blade and photosensitive member. For example, Japanese Patent Application Laid-Open No. 8-123279 discloses a cleaning assembly for reducing a frictional force. Japanese Patent Application Laid-Open No. 5-88597 discloses a cleaning assembly that can prevent uneven contact or faulty cleaning.
In recent years, because of higher requirements for copied images, it is earnestly sought to establish a technique which can stably provide high image quality. Copying machines of different types are increasingly required to be made more highly precise, more high-speed, more digital, smaller-sized and more low-cost. Under such circumstances, with regard to the higher precision, toners are being made to have smaller particle diameter, and it has become popular to use toners of 0.005 to 0.008 mm in weight-average particle diameter as measured with a Coulter counter or the like.
With regard to the higher speed, such toners with small particle diameter must be improved in their fixing performance, but, as a conflicting fact, it becomes likely that melt-adhesion to photosensitive member (a phenomenon that components of a toner stick to the photosensitive member surface to cause faulty images) occurs. Also, the fact that the particle diameter is small is in itself in the direction of being disadvantageous to the melt-adhesion.
More specifically, when the toner with small particle diameter is removed by cleaning, the contact pressure of a blade must be changed so that the toner can be prevented from slipping through the blade. This, however, may result in a larger frictional force concurrently with the high-speed process, thus the matter can be said to be causative of the melt-adhesion.
Once the toner has melt-adhered to the photosensitive member surface, no image is formed at the melt-adhered areas because no imagewise exposing light is transmitted there, and this appears as minute black dots on the images. Also, once the melt-adhesion has occurred, even if it does not appear on the copied images at the initial stage, the melt-adhesion grows in the rotational direction with repetition of copying and come to cause line-shaped faulty images. Different from photosensitive members that are changed for new ones after copying on tens of thousands of sheets as in the case of organic photosensitive members, a-Si photosensitive members run on for a greater number of sheets. Accordingly, the occurrence of faulty images due to melt-adhesion is a problem that can not be ignored. To remove the grown melt-adhered toner, it is effective to abrade the surface with alumina powder or the like. In practice, however, the photosensitive member is changed for new one which results in a greatly high running cost. Hence, in order to advance the lower running cost, it is important to prevent the melt-adhesion of toner from occurring and growing.
With regard to this phenomenon of melt-adhesion of toner to the photosensitive member surface, the friction between the blade and the photosensitive member is considered to be a cause. Under the existing conditions, however, its detailed mechanism has not been found.
In order to prevent the melt-adhesion of toner, measures may be taken such that the blade is made to have a high hardness so that its ability to scrape off the toner having adhered can be improved, and the surface of the photosensitive member is modified so that the toner may adhere to the photosensitive member with difficulty. However, in the case where the blade is made to have a higher hardness, the properties of blade materials approach a state of glass from a state of rubber. Hence, the blade is improved in the ability to scrape off the toner but comes to be brittle, and it has become likely that the blade is broken to cause faulty cleaning. Also, in the case where the photosensitive member surface is modified, the frictional force may increase with an increase in process speed, resulting in an increase in abrasive force. Thus, it has become likely that the surface of the photosensitive member is scraped however it is modified and made highly effective, unless materials are selected carefully.
Accordingly, it has been sought to establish a copying process in which the photosensitive member surface has a high hardness and is not scraped even under sever conditions as in the high-speed process making use of toners with small particle diameter, and also may cause no melt-adhesion of toner and no deterioration of such function over a long period of time even when copies are taken on a great number of sheets.
It has also been sought to establish a copying process in which images with a high quality level can be formed over a long period of time, no faulty cleaning may occur, and a reduction of maintenance cost can be achieved because of a blade made to have a long service life and a photosensitive member made to have a long service life.